New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism

Abstract

The effect of pH on the hydrogen oxidation and evolution reaction (HOR/HER) rates is addressed for the first time for the three most active monometallic surfaces: Pt, Ir, and Pd carbon-supported catalysts. Kinetic data were obtained for a proton exchange membrane fuel cell (PEMFC; pH z 0) using the H 2 -pump mode and with a rotating disk electrode (RDE) in 0.1 M NaOH. Our findings point toward: (i) a similar z100-fold activity decrease on all these surfaces when going from low to high pH; (ii) a reaction rate controlled by the Volmer step on Pt/C; and (iii) the H-binding energy being the unique and sole descriptor for the HOR/HER in alkaline electrolytes. Based on a detailed discussion of our data, we propose a new mechanism for the HOR/HER on Pt-metals in alkaline electrolytes.Fuel cells and electrolyzers are important for renewable energy conversion and storage. They are currently based on protonexchange membranes (PEMs) operating at low pH (pH z 0), which offer high power densities, but require large amounts of platinum for the oxygen reduction reaction (ORR) in fuel cells 1 and of Ir for the oxygen evolution reaction (OER) in electrolyzers. 2 For the hydrogen oxidation/evolution reaction (HOR/HER) only very small amounts of Pt are required due to its extremely high activity for the HOR/HER. 3 The H 2 anode performance in PEMFCs suggested exchange current densities (i 0 ) in the order of 10 2 mA cm Pt À2 , 4 which was conrmed by mass-transport-free fuel cell measurements 3,5 and microelectrode data. 6 Until then, 100-fold lower i 0 -values for Pt in acid were reported erroneously, generally based on rotating disk electrode (RDE) measurements 7,8 from which, however, the kinetics of reactions with i 0 -values much above the diffusion limited RDE current density (z2-3 mA cm disk À2 ) cannot be quantied. 9In an alkaline electrolyte, non-noble metal catalysts are very active for the ORR 10,11 and for the OER, 12,13 so that in conjunction with alkaline membranes (OH À -exchange membranes 14,15 ) a replacement of the noble-metal intensive PEM technology by alkaline membrane technology seems promising. Unfortunately, for yet unclear reasons, the HOR/ HER kinetics on Pt are much slower in alkaline than in acid electrolytes, and large amounts of Pt are needed to catalyze the HOR/HER in an alkaline environment. 9 Therefore, it is critical to develop alternative HOR/HER catalysts for alkaline electrolytes and -to guide this search -to elucidate the reasons for the poor HOR/HER activity of Pt in alkaline electrolytes.Traditionally, the overall reactions have been written either with protons in acid or with hydroxide ions in alkaline media: 16 in acid:in base:The future of electromobility relies on the development of cost effective and durable energy conversion systems such as fuel cells and electrolyzers. These devices, based on proton-exchange membranes (PEMs), operating at pH 0, offer high power densities, but require large amounts of noble metal for the oxygen reduction reaction (ORR) in fuel cells and the oxygen evo...

Pd/Ni Synergestic Activity for Hydrogen Oxidation Reaction in Alkaline Conditions

“…The Tafel plots show both the Pd/Ni investigated herein and the Pd/C reported in reference. 4 For high Pd coverage on Ni, the exchange current reaches the same value as the Pd/C electrocatalyst. This low values (<0.1 mA/cm 2 Pd ) seem to be typical of pure Pd in alkaline conditions and has been corroborated by the study of Pd covered Au surface.…”

“…However, if part of the results tend to corroborate the oxophilic effect in bifunctional bimetallic catalysts, [30][31][32] some recent results on monometallic oxophilic metals like Ir, show the same trend of HOR decreasing activity from acidic pH to alkaline pH as a non-oxophilic catalyst like Pt. 4 On the other hand, examples have been given for the beneficial effects of surface alloying to HOR rate enhancement in alkaline media, providing both explanations, one based on electronic effects modifying the HBE 33 and the other on added oxophilic function. 32 Such a conclusion may be true for a single metal but the extrapolation to bimetallic surfaces like the Pd/Ni reported in the manuscript may not be accurate.…”

“…In one of the few studies investigating the HOR activity of platinum in both acidic and alkaline media for several PGMs (Pd, Ir and Pt), the HOR kinetics were found to be at least two orders of magnitude slower in alkaline electrolyte than in acid electrolyte. 4 On the other hand, AMFCs allow the use of non PGMs catalysts in the less aggressive medium (alkaline, pH = 13) where most of the transition metals form stable solid species. 5 Therefore, the combination of a very low content of active PGM with a low-cost transition metal is very appealing as HOR catalyst in AMFCs.…”

Hydrogen Electrooxidation under Conditions of High Mass Transport in Room-Temperature Ionic Liquids and the Role of Underpotential-Deposited Hydrogen

“…The contributions of two different forms of H upd (the strongly-bound, H SB , and weakly-bound, H WB forms) are considered, and i represents these two different forms. In order to use Equation 13, the coverage of H upd as a function of overpotential in our PIL had to be known and in Figure 5 we have deconvoluted the H upd region into H SB and H WB sites, which give the parameter θ in Equation 13. As very little is currently known about H upd adsorption in PILs, the curve fits were deliberately kept as simple as possible, allowing only two peaks with normal distributions.…”

“…[4][5][6][7][8][9][10][11][12][13] The Tafel-Heyrovsky-Volmer mechanism of the HOR in acidic medium contains three elementary steps: 14 …”